Naturat Irensiata: Our natural heritage

This is part three of a five part series covering wildlife and fish species and habitats of greatest conservation concern in the Commonwealth. The last two weeks we brought you the Mariana fruit bat and bird species of our Commonwealth islands, and today, we bring you the coconut crab—the world’s largest land crab.

The Division of Fish and Wildlife is updating and refining the list of species and habitats of greatest conservation concern as part of the Comprehensive Wildlife and Fish Conservation Plan for the CNMI. We request public comments regarding this species or any other marine or land species or habitats that you believe should be addressed through management or conservation actions. Let us know your thoughts by email: CNMI_wildife@msn.com, or mail: Comments, PO Box 506369, Saipan, MP 96950, or attend our public meetings during April (dates will be posted later). Si Yu’us Ma’ase!

Ayuyu: The world’s largest land crab

We have the world’s largest land crab, the coconut crab living right here on our Commonwealth islands! You may not have seen them (like we often see hermit crabs), because they hide during the day and forage at night. Known as the ayuyu in the Chamorro language, our coconut crab is historically important to the Chamorro and Carolinian cultures. This remarkable crab can live to at least 40 years old. At full maturity, adult males can weigh in excess of 9 to 11 pounds (4-5 kg) and have a leg span of up to three feet (90 cm)! The ayuyu use both the land and the sea. Their lives begin in our coastal waters where females release their eggs into ocean currents under the cover of darkness. Their eggs hatch into tiny larvae that drift with currents for 2-3 weeks before settling on the bottom where they transform into tiny crabs (refer to the life stage diagram). These young crabs find small seashells (marine snail shells) that they wear for protection, just like hermit crabs. Once protected, they move into the higher inter-tidal areas, seeking shelter among rocks and cobble. They discontinue using seashells once their own carapace (shell) forms (usually before they reach 0.4 inches in length). In comparison, hermit crabs require increasingly larger seashells for protection throughout their lives (hence the value of leaving sea shells on the beach).

Coconut crabs crawl inland as much as 2.5 miles (4 km) where they inhabit forests for protection and food. They grow slowly, molting (shedding) their carapace shell as a new carapace forms. Molting occurs 2-3 times per year for small crabs (<0.8 inches [20 mm] thoracic length), and once per year for larger crabs (>1 inch [25 mm] thoracic length). The largest crabs may not molt every year.

The ayuyu eat a variety of fruits, decaying plant matter, and small or dead animals. Of course, the coconut is one of their favorite fruits, which they selectively pry open using their large, strong claws

The ayuyu’s population is adversely impacted by introduced animals (such as rats, wild pigs, monitor lizards, and dogs), over-harvesting, and deforestation. They are a game species with a hunting season and limits on the number taken, and a hunting license is required. They only can be taken by hand and only if their carapace is more than 3 inches (7.5 cm) wide (at approximately 10 years of age). The limit is 5 per day and 10 per person per hunting season. It is illegal to sell coconut crabs in the CNMI, or to take females with eggs.

The need for the females and their young to cross roadways to and from the ocean also makes them vulnerable to motor vehicle road kill, particularly where busy roadways parallel our coasts. Portions of the population can be eliminated from our islands though the years as a result of these cumulative roadway impacts. The long-term toll of these impacts, as well as others, is uncertain and studies are needed to assess them. Funding limitations have prevented detailed research of the ayuyu; hence population estimates and trends are uncertain. However, hunters and others state that the populations are declining and the adult sizes are decreasing through the years. We are fortunate to still have this unique member of our island home. The ayuyu is an important part of our natural and cultural heritage, and it requires conservation awareness and management to sustain them for future generations.

Information derived from Stock Assessment of Coconut Crabs—Vanuatu by W. Fletcher and M. Amos and from DFW files. (Greg Schroer)

(Greg Schroer is a consulting senior scientist and planner at the Division of Fish and Wildlife, Department of Lands and Natural Resources.)